About accessibility

This page describes how this website has been made more accessible to people with disabilities and to a wide range of devices for the web.

Hearing disabilities

When our site delivers sound or video, we additionally provide captioning and transcripts to provide access to deaf and hard-of-hearing people.

Visual disabilities

People with visual impairments or disabilities need certain accommodations to use websites easily and effectively. For example, some use a screen reader, which identifies what is being displayed and presents it to the user in a form other than a display. Or, some may need to view the site at larger font sizes. The following features of this website support their needs:

All pages in this site include "skip navigation" links, which enable people using linear delivery user agents to skip through page navigation sections more easily. These links are not displayed to users without visual disabilities, but they are accessible to devices such as screen readers and to those who use the keyboard for primary navigation.

All images include alt tags to provide descriptions to user agents that don't support images or that have images turned off. All images with unimportant content and all images used as spacers for layout use a null string in the alt tag. All graphical bullets use an alt tag of "Item: " or are rendered using list-style-image cascading style sheets (CSS) properties.

All text on this site can be embiggened using zoom features found in modern web browsers.

All text on this site is displayed with high contrast between the text foreground and background colors. In some cases this contrast is intentionally deemphasized when the information is not the main focus of the page, and the user can increase the contrast dramatically by hovering the cursor in the area. Examples: the header, footer, and left navigation.

Captions and transcripts for video describe all relevant non-speech information.

Motor disabilities

People with motor or mobility impairments or disabilities need certain accommodations to use websites easily and effectively. For example, some use specially designed input devices if a finger, hand, or arm has limited or no ability.

The following features of this website support their needs:

All forms on this site use accesskey and tabindex attributes, enabling swift keyboard access to form fields and ensuring a predictable tab order among fields.

Some links use the title attribute to provide more information about a link before you select it, enabling you to make better judgments about choosing a link before making a physical commitment to choosing it.

All pages on this site include link tags such as Home, Search, Glossary, and Copyright, which provide standardized navigation in user agents that support them. User agents can provide these links in a consistent location on the page and also within their own interfaces (e.g., menus). They may also have keyboard shortcuts for these special links.

Device independence

Most people view the web with a graphical web browser on a desktop or laptop computer with a color screen display of 1024 by 768 pixels or larger. However, the web is not just for those web browsers and just those display resolutions. Other ways to use the web are: iPads and other subcompact computers, handheld computers, web browsers on smartphones, text-based web browsers, screen readers, aural browsers, refreshable Braille devices, and more.

The following features of this site support device independence to enable greater accessibility to a wider range of devices:

All pages in this site use heading tags correctly for structure rather than presentation.

This site doesn't use tables for layout.

This site doesn't use Adobe Portable Document Format (PDF) files as a primary method of delivering content.

This site uses cascading style sheets to separate structure from presentation.

When user agent support for cascading style sheets is unavailable or disabled, all the same content is still available.

Validation and compliance

All pages in this site are validated against a validation service such as the W3C Markup Validation Service or the Web Design Group Validator to ensure compliance with W3C markup standards.

This site aims to provide Level AA compliance of the Web Content Accessibility Guidelines 1.0 provided by the World Wide Web Consortium.

This site aims to provide full compliance of the Section 508 guidelines provided by the United States federal government.

This site approaches or meets all 3 of these specifications for most pages, but does not yet claim full compliance for all pages at all levels.

Known problems

With stylesheets disabled, the Mission Bay tour (slideshow) is inaccessible.

Planned improvements

This site currently provides many accessibility features, but it also has plenty of room for improvement. To further enhance the accessibility of this site, we plan to:

Implement more accessibility support for data tables using id, headers, abbr, and scope attributes for td and th elements.

Ensure that all links make sense when read out of context to provide more support for user agents that provide a list of the page's links separated from the rest of the page's content.

Many pages of this site could be improved to provide better support for abbr and acronym tags, enabling devices such as screen readers to understand, for example, that "CSS" means "cascading style sheets" or "ns" means "nanoseconds" thereby providing proper meaning to what would otherwise be an unpronounceable nonsense word.